Power supply apparatus with overload protection, referred to herein as a ‘smart power switch’, is used in many applications, including automotive equipment, especially to control vehicle lighting, for example. The present invention is particularly, but not exclusively, usable in such applications.
Smart power switches are often required to include an automatic re-start capability, that is to say, for re-starting the switch if the overload is not excessive. In operation, the smart power switch switches itself to the OFF-state in response to an overload. If the overload is not excessive in duration and magnitude so that it is not dangerous for the switch itself or for the wiring and other system components, the smart power switch automatically turns itself to the ON-state again after a short interval, providing the opportunity to recover full power if the overload was sporadic or transient, and providing a degree of emergency power if the cause of the overload condition persists.
Integrated smart power switches may use various protection strategies in response to an overload condition, taking account of the characteristics of switching a capacitive load or an incandescent bulb, for example, both of which exhibit a high initial current ‘inrush’ before the current then settles to a lower steady magnitude.
One common strategy is an over-temperature shutdown, which protects the switch in case of high power dissipation in the switch, especially at high ambient temperatures. A problem encountered with this strategy is that it does not protect against a possible moderately high permanent junction temperature which degrades and finally destroys the semiconductor switch.
Another protection strategy uses a linear current limitation which limits the switch current to safe values. This strategy introduces a high amount of power dissipation in the switch during the turn-on phase of a capacitive load or a incandescent bulb. Therefore the switch must be deactivated by a supervising intelligence in case of overload (e.g. output short-circuit) to prevent it from destruction.
Yet another protection strategy uses an over-current shutdown feature with one or more current levels dependent on the load type. In case of a serious or a sporadic failure condition the switch is deactivated by the over-current shutdown feature. Therefore an intelligent control unit, such as a micro-controller is necessary to supervise the switch and reactivate the switch only in case of a sporadic failure condition to provide high availability of the output.
All these protection strategies suffer from the problem that the supervising intelligence of a micro-controller is necessary to provide both a high availability of the switch and also protect the switch. In case of failure of this micro-controller the switch is no longer safe and can be destroyed by an overload condition. Also, such an intelligent control unit is expensive.